rust-lang / rust-lang/rust

[ICE]: `index out of bounds: the len is 0 but the index is ..`

Open
#160,800 1 comment 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

C-bug F-type_alias_impl_trait I-ICE needs-triage T-compiler
Dominant language
Rust
Stars
119k
Forks
16.1k
PR metrics pending

Description

Code
#![feature(type_alias_impl_trait)]
type Foo = impl Send;

struct A;

#[define_opaque(Foo)]
const VALUE: Foo = todo!();

#[define_opaque(Foo)]
fn test(foo: Foo) {
    match VALUE {
        &mut Some(ref mut x) => *x,
        &mut None => panic!(),
    }
}

fn main() {}
Meta

rustc --version --verbose:

rustc 1.99.0-nightly (771916f90 2026-08-08)
binary: rustc
commit-hash: 771916f9028e7fe56d2685f2c4f698de5d7d6a45
commit-date: 2026-08-08
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 23.1.0
Error output
error: item does not constrain `Foo::{opaque#0}`
 --> code.rs:7:7
  |
7 | const VALUE: Foo = todo!();
  |       ^^^^^
  |
  = note: consider removing `#[define_opaque]` or adding an empty `#[define_opaque()]`
note: this opaque type is supposed to be constrained
 --> code.rs:2:12
  |
2 | type Foo = impl Send;
  |            ^^^^^^^^^

warning: unused variable: `foo`
  --> code.rs:10:9
   |
10 | fn test(foo: Foo) {
   |         ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
   |
   = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default
Backtrace 

thread 'rustc' (3960277) panicked at /rustc-dev/771916f9028e7fe56d2685f2c4f698de5d7d6a45/compiler/rustc_mir_dataflow/src/un_derefer.rs:67:20:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
   0:     0x7f174d70d2b6 - <<std[d336e1aa20b017e8]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[f3eec57d7c5a5c99]::fmt::Display>::fmt
   1:     0x7f174de11c8f - core[f3eec57d7c5a5c99]::fmt::write
   2:     0x7f174d7221dc - <std[d336e1aa20b017e8]::sys::stdio::unix::Stderr as core[f3eec57d7c5a5c99]::io::write::Write>::write_fmt
   3:     0x7f174d6e032a - std[d336e1aa20b017e8]::panicking::default_hook::{closure#0}
   4:     0x7f174d701043 - std[d336e1aa20b017e8]::panicking::default_hook
   5:     0x7f174c65beda - std[d336e1aa20b017e8]::panicking::update_hook::<alloc[a6218fca04c82945]::boxed::Box<rustc_driver_impl[7956faa26824032a]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f174d7014e2 - std[d336e1aa20b017e8]::panicking::panic_with_hook
   7:     0x7f174d6e03e2 - std[d336e1aa20b017e8]::panicking::panic_handler::{closure#0}
   8:     0x7f174d6d92b9 - std[d336e1aa20b017e8]::sys::backtrace::__rust_end_short_backtrace::<std[d336e1aa20b017e8]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f174d6e1dbd - __rustc[9438f7ed0802b081]::rust_begin_unwind
  10:     0x7f174a37f46c - core[f3eec57d7c5a5c99]::panicking::panic_fmt
  11:     0x7f174ae5c154 - core[f3eec57d7c5a5c99]::panicking::panic_bounds_check
  12:     0x7f174a14767a - <rustc_mir_dataflow[69773cb1756060d8]::impls::initialized::MaybeInitializedPlaces as rustc_mir_dataflow[69773cb1756060d8]::framework::Analysis>::iterate_to_fixpoint
  13:     0x7f174a16761d - <rustc_mir_transform[28b060d57b516d55]::elaborate_drops::ElaborateDrops as rustc_mir_transform[28b060d57b516d55]::pass_manager::MirPass>::run_pass
  14:     0x7f174de17de2 - rustc_mir_transform[28b060d57b516d55]::run_analysis_to_runtime_passes
  15:     0x7f174e4d0717 - rustc_mir_transform[28b060d57b516d55]::mir_drops_elaborated_and_const_checked
  16:     0x7f174e4d38f5 - rustc_query_impl[f2c47540efc44da2]::execution::try_execute_query::<rustc_data_structures[47e0eddda0e0a292]::vec_cache::VecCache<rustc_span[5a330e2a527940b1]::def_id::LocalDefId, rustc_middle[78cd89ce21bdaa52]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[78cd89ce21bdaa52]::dep_graph::graph::DepNodeIndex>, false>
  17:     0x7f174e4d3103 - rustc_query_impl[f2c47540efc44da2]::query_impl::mir_drops_elaborated_and_const_checked::execute_query_non_incr::__rust_end_short_backtrace
  18:     0x7f174e4d622a - <rustc_middle[78cd89ce21bdaa52]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[fca7d0b553b989d0]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  19:     0x7f174e4d533e - rustc_interface[fca7d0b553b989d0]::passes::analysis
  20:     0x7f174efa808a - rustc_query_impl[f2c47540efc44da2]::execution::try_execute_query::<rustc_middle[78cd89ce21bdaa52]::query::caches::SingleCache<rustc_middle[78cd89ce21bdaa52]::query::erase::ErasedData<[u8; 0usize]>>, false>
  21:     0x7f174efa7cff - rustc_query_impl[f2c47540efc44da2]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  22:     0x7f174f02cb35 - rustc_interface[fca7d0b553b989d0]::interface::run_compiler::<(), rustc_driver_impl[7956faa26824032a]::run_compiler::{closure#0}>::{closure#2}
  23:     0x7f174f06d424 - std[d336e1aa20b017e8]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[fca7d0b553b989d0]::util::run_in_thread_with_globals<rustc_interface[fca7d0b553b989d0]::util::run_in_thread_pool_with_globals<rustc_interface[fca7d0b553b989d0]::interface::run_compiler<(), rustc_driver_impl[7956faa26824032a]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  24:     0x7f174f06d1ed - <std[d336e1aa20b017e8]::thread::lifecycle::spawn_unchecked<rustc_interface[fca7d0b553b989d0]::util::run_in_thread_with_globals<rustc_interface[fca7d0b553b989d0]::util::run_in_thread_pool_with_globals<rustc_interface[fca7d0b553b989d0]::interface::run_compiler<(), rustc_driver_impl[7956faa26824032a]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[f3eec57d7c5a5c99]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  25:     0x7f174f0a56ba - <std[d336e1aa20b017e8]::sys::thread::unix::Thread>::new::thread_start
  26:     0x7f1748497739 - <unknown>
  27:     0x7f174851bedc - <unknown>
  28:                0x0 - <unknown>

error: the compiler unexpectedly panicked. This is a bug

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/home/matthias/vcs/github/CRED/rustc-ice-2026-08-09T10_49_26-3960267.txt` to your bug report

note: rustc 1.99.0-nightly (771916f90 2026-08-08) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_drops_elaborated_and_const_checked] elaborating drops for `test`
#1 [analysis] running analysis passes on crate `code`
end of query stack
error: aborting due to 1 previous error; 1 warning emitted

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the ICE with the supplied type_alias_impl_trait example on the reported nightly, then start at compiler/rustc_mir_dataflow/src/un_derefer.rs:67 and follow the MaybeInitializedPlaces analysis through MIR drop elaboration. Use the query stack to trace the failure in test; done means this input no longer panics with an index-out-of-bounds ICE and the compiler reports the issue normally.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Quiet
Clarity
Mostly clear
Newbie friendliness
45/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.